Electrospark-deposit coatings have been recognized to be amongst the most damage-resistant coatings that are suitable for use in harsh environments [Electrospark Deposition for Depot- and Field-Level Component Repair and Replacement of Hard Chromium Plating, Final report Project WP-0202 (page 11), Sep. 7, 2006, Environmental Security Technology Certification Program (ESTCP)]. Unfortunately, similar to all other coating technologies, conventional electrospark deposition (ESD) has its own limitations and problems. One of these is the process's weakness in coping with complex surface geometries and internal surfaces, in particular, those with small features. The limitation arises because ESD normally employs a rigid solid, sizable consumable electrode (FIG. 1). Another problem of the process is the occurrence of unstable electrospark discharges. Unstable discharging will seriously undermine the quality of the coating. Despite being a difficult problem, many researchers have taken up this challenge to come up with better process designs in order to overcome the instability problem. A noticeable development was the work at the Pacific Northwest National Laboratory and the U.S. Army Armament Research Center [R. N. Johnson, J. A. Bailey, Joseph A. Goetz, Electro-spark deposited coatings for replacement of chrome plating, Armament Research, Development and Engineering Center, New Jersey, Report ARAET-CR-05002, June (2005)]. At there, researchers developed a sophisticated ESD system that can control the contact force precisely. The system is capable of controlling the electrode force to ±5 g during processing, an achievement claimed to be difficult to accomplish even by major robotics manufacturers. With such an advanced but expensive deposition system, good quality coatings can be produced.
In the US patent application publication 2012/0193329, Liu et al. makes use of powder as a feeding stock, but it does not involve any magnetic devices/forces. The powder simply passes through a “feed channel configured within or at least partially surrounding the electrode for guiding powder” (FIG. 2).
A related research paper on the use of loose powder in electrodischarge applications published by Reynaerts et al. [J. Qian, S. Steegen, E. Vander Poorten, D. Reynaerts, H. Van Brussel, EDM texturing of multicrystalline silicon wafer and EFG ribbon for solar cell application, International Journal of Machine Tools & Manufacture 42 (2002) 1657-1664] has described the employment of magnetized metal powder to surface texturing of Si substrates using electrodischarge machining. The aim was to roughen Si wafers and ribbons for solar cell application. In other words, Reynaerts et al. only teaches the electrodischarge technique of using metal powder functions as a thermal erosion process (subtractive) rather than an additive deposition method as in the present invention.
In another related research paper on the subject of using loose powder materials for ESD by Topal{hacek over (a)} et al [Pavel Topal{hacek over (a)}, Laureniu Sl{hacek over (a)}tineanu, Oana Dodun, Margareta Cotea{hacek over (a)}, and Natalia Pînzaru, Electrospark deposition by using powder materials, Materials and Manufacturing Processes 25 (2010) 932-938]. Topal{hacek over (a)} et al. describes the employment of a relatively high voltage (up to 12 kV) between the electrode and the workpiece to initiate electrical discharges across a relatively large spark gap, without direct contact having been made between the electrodes. Once a discharge plasma channel has formed, the powder for alloying or coating is then fed into the channel (FIG. 3), so as to melt and deposit the material on the external surface of a cylinder. This process has the disadvantages of needing to use a relatively high voltage, therefore high energy, and the process is unsuitable for processing parts with complex surface contours and shapes. For this process the anode is not magnetized and does not behave like a soft brush.